We found that boys with higher scores on the Child Psychopathy Scale showed smaller skin conductance responses (SCRs) to unsignaled and nonaversive auditory stimuli. To our knowledge, this is the first study to show that psychopathic traits are associated with lower reactivity (SCR hyporeactivity) to orienting stimuli in a community sample of children. Reduced electrodermal orienting has been reported in children with conduct disorder (Herpertz et al., 2001
; Delamater & Lahey, 1983
), but not specifically in relation to psychopathic personality traits. Interestingly, the association was mediated by the interpersonal (manipulative/deceitful) factor of psychopathy; neither callous/disinhibited traits nor conduct problems were linked to hyporeactivity in boys. Furthermore, psychopathy did not predict hyporeactivity in girls.
Reduced electrodermal reactivity in higher-scoring boys was found across a wide range of stimuli, including tones, speechlike stimuli, and miscellaneous bird sounds. With the exception of baby cries, there was no evidence that hyporeactivity was confined to ‘novel’ sounds or only to the first trial of a stimulus. This finding is inconsistent with some of the literature on psychopathy-proneness and reactivity to nonsignal stimuli. Most orienting tasks have utilized a habituation series, in which a neutral tone is presented repeatedly until the electrodermal system ceases to respond. Several investigators have found that psychopathy-prone juveniles can be characterized by lower SCR amplitude only to the first stimulus (Borkovec, 1970
; Schmidt et al., 1985
). In the present study, by contrast, the hyporeactivity effect persisted across multiple presentations of tones.
Reduced electrodermal activity to nonsignal stimuli is thought to reflect deficits in attentional processes (Raine & Venables, 1984
; Fowles, 2000
), which may in turn be related to inferior functioning of the prefrontal cortex. Antisocial individuals with schizotypal tendencies (who demonstrably suffer cognitive and affective impairments) are exceptionally hyporesponsive to orienting stimuli (Raine, 1993
). In the present study, the hypothesis of prefrontal dysfunction was not supported. If prefrontal impairment is responsible for hyporeactivity, then we would have expected to obtain significant relations with callous/disinhibited traits (impulsiveness, behavioral dyscontrol, lack of planning, etc.). This was not the case, as only the Manipulative/Deceitful factor was associated with reduced SCR magnitude. It should be noted, however, that prefrontal regions develop over the course of adolescence, leading to the possibility that prefrontal-related orienting deficits have yet to manifest in our young sample.
The callous/disinhibited features of psychopathy, unlike the interpersonal facets, were associated with greater internalizing problems. This supports the view that a disabling ‘secondary’ syndrome, characterized by poor impulse control and higher anxiety, can coexist with the more socially dominant, ‘primary’ aspects of psychopathy (Lykken, 1995
). An important caveat is that ‘secondary’ psychopathy is not traditionally thought to include callous-unemotional traits, whereas factor analyses in the present sample indicated that ‘callousness’ and ‘poverty of affect’ share much in common with impulsive/disinhibited traits. Conversely, ‘failure to accept responsibility’ loaded on the Manipulative/Deceitful factor, which might reflect an element of verbal persuasiveness that facilitates the shifting of blame. Furthermore, the ‘grandiosity’ and ‘lack of guilt’ subscales failed to load on either of the two main factors. These unexpected patterns may be due to psychometric limitations in the Child Psychopathy Scale, or may be a product of the specific developmental (i.e., preadolescent) status of our subjects.
Recent empirical investigations have argued for the separation between callous-unemotional and deceitful interpersonal traits (Hare, 2003
; Cooke & Michie, 2001
). This has important implications in the framing and interpretation of psychophysiological results. Although the particular stimulus contexts and psychopathy facets that underlie electrodermal hyporeactivity are not straightforward (Benning et al., 2005
; Patrick, Cuthbert, & Lang, 1994
), studies that have employed aversive stimuli have typically interpreted hyporeactivity as evidence of an emotional disturbance (Blair, 1999
). However, it may be necessary to separate the interpersonal (i.e., deceitful and charming) features from the affective (i.e., emotionally detached) features when making inferences about the underlying personality processes. For example, Verona et al. (2004)
found that prison inmates who scored high on Factor 1 of the PCL-R (Hare, 1991
) had deficient electrodermal responses to emotionally-charged acoustic stimuli. Upon further examination, it appeared that the interpersonal facet, rather than emotional detachment, was related to the criminals' failure in differentiating affective from neutral sounds. This result, coupled with the present findings, suggest that glibness and manipulation should be more carefully examined in relation to electrodermal deficits. It also fuels the speculation that the verbal components of psychopathy (i.e., glibness and manipulation) are related to atypical processing of verbal/acoustic stimuli (Raine & Venables, 1988
Physiological investigations of psychopathy have relied primarily on male or institutionalized samples. The present study is unique in that it examined a mixed-gender sample of preadolescent twins. We found an interaction of sex and psychopathy level, such that only boys with higher psychopathy scores were electrodermally hyporeactive. This challenges an intuitive explanation regarding the nature of gender differences. According to some theories (e.g. Mednick et al., 1977
), biological correlates of antisocial personality should be stronger in females because the ‘social push’ is less prominent. That is, a greater dosage of biological risk factors is necessary to steer a female in a psychopathic direction.
Although few studies have investigated the electrodermal correlates of psychopathic traits in females, a sex-specific biological association is not unique in the psychopathy literature. At the endocrinal level, for example, there is evidence that cortisol level is lower in adolescent males who have a high degree of psychopathic traits (Loney et al., 2006
); this association is absent in high-scoring females. Furthermore, the cortisol stress response appears to be uniquely elevated in males with low psychopathy scores (O'Leary, Loney, & Eckel, 2007
). Females, regardless of their psychopathy score, show a pattern of cortisol reactivity that is similar to the high-scoring males.
This result echoes the present finding that the electrodermal responses of higher-scoring males were not deviantly low, but were simply diminished relative to their lower-scoring male counterparts. In other words, there was indication that boys with lower psychopathy scores were actually responding to the auditory stimuli in a hyperreactive manner - much higher than that observed in girls. This implies that electrodermal hyperreactivity may be a sex-specific protective factor that shields males from the development of a manipulative-deceitful orientation.
A major strength of this study is methodological, in that twin data provides an optimal method of handling potentially noisy or, at worst, confounding variables. Multilevel models indicated that the hyporeactivity phenomenon was largely due to effects occurring within twin pairs, rather than mediated by between-family variables. The latter is plagued by much noise, given that the SCR signal is affected by anatomical properties of the skin as well as atmospheric conditions (Boucsein, 1992
; Venables & Mitchell, 1996
). Measurement error is reduced at the within-pair level because gross anatomical features (e.g., skin pigmentation) and environmental factors (e.g., temperature/season) are necessarily held constant across twins
The absence of a between-pair source of the hyporeactivity phenomenon may also be due to measurement error in the assessment of psychopathic traits. Different caregivers may have disparate standards, as well as differential susceptibility to social desirability biases, when rating their children. This underscores the fact that a subject's absolute score is less reliable than his relative score (vis-à-vis his sibling). The within-pair deviation in CPS scores effectively boils down to an observational measure (i.e., observations based on a standard rating system), perhaps explaining why hyporeactivity was found only at the within-family level.
Limitations and future directions
It should be noted that the orienting paradigm employed in this study is quite different from that used by previous investigators. In addition to the use of highly novel stimuli, several sounds approached 105 decibels. This is an unusually high intensity for stimuli designed to elicit orienting responses. The implication, therefore, is that reduced SCR magnitude may represent poor defensive reactions as opposed to deficient orienting responses. In adult psychopaths, for example, hyporeactivity was found for 120-dB tones but not for tones at lower intensities (Hare, 1978
). Similarly, in children with conduct disorder, hyporeactivity to an initial 90-dB bell sound was found, but not for a 75-dB tone (Schmidt et al., 1985
). Nevertheless, it should be noted that the effects found here with the more intense novel sounds presented in Block 3 were also found with less intense tones.
Our findings await replication using an expanded range of stimulus conditions and valences. It will be interesting to see if electrodermal hyporeactivity is attendant to blatantly aversive stimuli in addition to orienting stimuli. In the meantime, this study suggests that the interpersonal facet may be a focal point for uncovering the biological bases of psychopathy in males. It contributes to the emerging perspective (O'Leary et al., 2007
; Justus & Finn, 2007
) that the biological correlates of psychopathy may be sex-specific.